I would recommend a good flush out with a de greaser first, even leaving it in the tank to soak in for a couple of days.
secure a small brush on to a rod so you can clean the inside as much as possible.
Using a steam cleaner is a good idea for old tanks.
If you think that there is rust flaking off inside then check with your local trail bike shop, there has been a product out for a while that seals the inside of rusty tanks
It may also be a good idea to use a sedamant bowl on the outlet of the tank to catch any initial gunk before it gets to the filters.
Good Luck
Roscoe